The field of the invention relates to materials that are non-linear optical compounds with a general chemical formula
(xcexa3iMxcex1i1)(xcexa3jMxcex2j2)(xcexa3kMxcex3k3)Be2O5xe2x80x83xe2x80x83Formula 1
wherein M1, M2, and M3 are mono-, di-, or tri-valent metal ions respectively; wherein (xcexa3ixcex1i)=X and ranges from 0 to 6, (xcexa3jxcex2j)=Y and ranges from 0 to 3, and (xcexa3kxcex3k)=Z and ranges from 0 to 2, (hereinafter referred to as xe2x80x9cMBe2O5xe2x80x9d compounds). Another embodiment of the present invention satisfies the generally formula
(xcexa3iMxcex1i1)Be2O5xe2x80x83xe2x80x83Formula 2
wherein M1 is a mono-valent metal ion; and wherein (xcexa3ixcex1i)=X=6; and yet another embodiment of the present invention satisfies the general formula
(xcexa3j=1-3Mxcex2j2)Be2O5xe2x80x83xe2x80x83Formula 3
wherein M2 is a di-valent metal ion; and wherein (xcexa3jxcex2j)=Y=3, another embodiment of the present invention satisfies the general formula
(xcexa3kMxcex3k3)Be2O5xe2x80x83xe2x80x83Formula 4
wherein M3 is a tri-valent metal ion; and wherein (xcexa3kxcex3k)=Z=2. Mono- and di-valent metal ions, M1 and M2, that are suitable for forming compounds satisfying the general formula are preferably independently selected from the group consisting of Groups IA and IIA, however other mono- and di-valent cations may be used so long as the material has a non-centrosymmetric atomic arrangement, (hereinafter referred to as xe2x80x9cMBE2O5xe2x80x9d compounds).
Nonlinear optical (NLO) materials are unusual in that they affect the properties of light. A well-known example is the polarization of light by certain materials, such as when materials rotate the polarization vectors of absorbed light. If the effect on the polarization vector by the absorbed light is linear, then light emitted by the material has the same frequency as the absorbed light. NLO materials affect the polarization vector of the absorbed light in a nonlinear manner. As a result, the frequency of the light emitted by a nonlinear optical material is affected.
More specifically, when a beam of coherent light of a given frequency, such as produced by a laser, propagates through a properly oriented NLO crystal having non-zero components of the second order polarizability tensor, the crystal will generate light at a different frequency, thus extending the useful frequency range of the laser. Generation of this light can be ascribed to processes such as sum-frequency generation (SFG), difference-frequency generation (DFG) and optical parametric amplification (OPA). Devices using NLO crystals include, but are not limited to, up and down frequency converters, optical parametric oscillators, optical rectifiers, and optical switches.
Frequency generation in NLO materials is an important effect. For example, two monochromatic electromagnetic waves with frequencies xcfx891 and xcfx892 propagating through a properly oriented NLO crystal can result in generation of light at a variety of frequencies. Mechanisms defining the frequency of light using these two separate frequencies are sum-frequency generation and difference-frequency generation. SFG is a process where light of frequency xcfx893 is generated as the sum of the two incident frequencies, xcfx893=xcfx891+xcfx892. In other words, SFG is useful for converting long wavelength light to shorter wavelength light (e.g. near infrared to visible, or visible to ultraviolet). A special case of sum-frequency generation is second-harmonic generation (SHG) where xcfx893=2xcfx891, which is satisfied when the incident frequencies are equal, xcfx891=xcfx892. DFG is a process where light of frequency xcfx894 is generated as the difference of the incident frequencies xcfx894=xcfx891xe2x88x92xcfx892. DFG is useful for converting shorter wavelength light to longer wavelength light (e.g. visible to infrared). A special case of DFG is when xcfx891=xcfx892, hence xcfx894=0, which is known as optical rectification. Optical parametric oscillation is also a form of DFG and is used to produce light at tunable frequencies.
The conversion efficiency of an NLO crystal for a particular application is dependent on a number of factors that include, but are not limited to: the effective nonlinearity of the crystal (picometers/volt [pm/V]), birefringence (xcex94n, where n is a refractive index), phase-matching conditions (Type I, Type II, non-critical, quasi, or critical), angular acceptance angle (radianxc2x7cm), temperature acceptance (xc2x0 C.xc2x7cm), walk-off (radian), temperature dependent change in refractive index (dn/dt), optical transparency range (nm), and the optical damage threshold (W/cm2). Desirable NLO crystals should posses an optimum combination of the above properties as defined by the specific application.
Borate crystals form a large group of inorganic NLO materials used in laser-based manufacturing, medicine, hardware and instrumentation, communications, and research studies. Beta barium borate (BBO: xcex2-BaB2O4), lithium triborate (LBO: LiB3O5), and cesium lithium borate (CLBO:CsLi(B3O5)2) are examples of borate-based NLO crystals developed in recent years that are being used widely as NLO devices, especially in high power applications. Select properties suitable for generation of laser light from the mid-infrared (IR) to the ultraviolet (UV) for these crystals are listed in Table 1.
BBO has a favorable non-linearity (about 2 pm/V), transparency between 2600 nm and 190 nm, significant birefringence (necessary for phase-matching), and a high damage threshold (15 GW/cm2, 1064 nm, 0.1 ns pulse width). However, its high birefringence creates a relatively small angular acceptance and a large walk-off angle that can limit conversion efficiencies. The crystal is relatively difficult to produce in large sizes and is somewhat hygroscopic.
LBO has good UV transparency (absorption edge=160 nm) and possesses a high damage threshold (25 GW/cm2, 0.1 ns, 1064 nm). However, it has insufficient intrinsic birefringence for phase-matching to generate deep UV radiation. Furthermore, LBO melts incongruently and must be prepared by flux-assisted crystal growth methods. This limits production efficiency that leads to small crystals and higher production costs.
CLBO appears to be a very promising material for high power production of UV light due to a combination of high nonlinearity and high damage threshold. The crystal can also be manufactured to relatively large dimensions. Unfortunately, the crystal is exceedingly hygroscopic and invariably sorbs water from the air; hence, extreme care must be taken to manage environmental moisture to prevent hydration stresses and possible crystal destruction.
With so many intrinsic physical parameters to optimize, known optical frequency converters, at present, are applicable to specific applications. A major factor limiting the advancement of laser applications is the inability of conventional NLO devices to generate laser light at desired wavelengths, power levels, and beam qualities. Currently-available NLO materials are not able to meet specifications required by many applications due to a number of factors that include: small nonlinear coefficients, bulk absorption in energy regions of interest, poor optical clarity, low damage thresholds, instability under operation, environmental degradation, difficulty in device integration, and high cost of manufacture. In many cases, the fundamental limit of conventional NLO materials has been met, and as a result, they are not able to meet specifications required by many present and future applications. Related properties and shortcomings are discussed in Chemistry of Materials, 1:492-508 (1989), Keszler, Curr. Opinion in Solid State and Mater. Sci. 1, 204 (1996); Becker Adv. Mater. 10(13) p. 979-992 (1998), which are hereby incorporated by reference.
At present, there are two UV NLO materials, one is xcex2-BaB2O4 (BBO), and the other KBe2BO3F2 (KBBF). BBO crystal has a planar (B3O6) group as the basic structure unit, and therefore, there is a conjugate xcfx80 orbital of non-symmetry in the valent orbitals of the structure that produces a high microscopic second-order susceptibility. The d22 coefficient, a major macroscopic NLO coefficient of BBO, is less than or equals to 2.7 pm/v, which is the highest in the ultraviolet NLO crystals currently known. However, there are shortcomings for BBO as an UV NLO crystal, some of which are listed below.
(1) The band gap of the structure is narrow so that the absorption edge of the crystal is about 189 nm, compared to about 170 nm for LBO. When BBO is used to produce a harmonic generation output in ranges from 200 nm to 300 nm, absorption is greatly enhanced compared to the visible range. This is why the crystal is easy damaged when used to produce a fourth harmonic generation with high fundamental optical power. In addition, owing to partial absorption of the quadruple frequency, the rise of temperature in irradiated crystals is inhomogeneous, which leads to a local change of refractive index and greatly falling of optical quality of the harmonic generation output;
(2) The birefringence of BBO xcex94n≅0.12, which is also related to the planar structure of B3O6 group arranged in the crystal lattice. This large birefringence of BBO makes the acceptance angle and walk-off ange at the frequency of quadruple multiplication to be too small (xcex94xcex8=0.45 mrad) to suit for device applications.
One possible way to overcome the above shortcomings of BBO by replacing the active NLO group B3O6 with BO3. The three oxygen terminals of BO3 should simultaneously bridge other atoms with the absorption edge shifting toward the blue side of spectrum, in the range of 150-160 nm. It is also possible for such a compound to reduce the birefringence, which favors an increased acceptance angle of the crystal. Based on these considerations KBe2BO3F2 (KBBF) was developed, with an absorption edge reaching 155 nm, birefringence down to about 0.7, and the phase-matchable range extending to 185 nm. However KBBF is difficult to grow because of the strong layer structure of the crystal lattice (the crystal appearance is similar to mica, with a severe cleavage at (001) plane of the lattice). This limits KBBF as a practical NLO material.
Because of the large number and diversity of present and projected applications, no single NLO material can be optimized for all uses. Thus far only a limited number of efficient NLO materials have been commercialized, thereby creating a bottleneck in the use of lasers in the advancement of many key technology areas. As a result, there is a continuing search for and development of new NLO materials.
It is an object of the present invention to produce and utilize nonlinear optical materials that satisfy the general formula
(xcexa3iMxcex1i1)(xcexa3jMxcex2j2)(xcexa3kMxcex3k3)Be2O5xe2x80x83xe2x80x83Formula 1
wherein M1, M2, and M3 are mono-, di-, or tri-valent metal ions respectively; wherein (xcexa3ixcex1i)=X and ranges from 0 to 6, (xcexa3jxcex2j)=Y and ranges from 0 to 3, and (xcexa3kxcex3k)=Z and ranges from 0 to 2, (hereinafter referred to as xe2x80x9cMBe2O5xe2x80x9d compounds).
Another object of the present invention is to produce a non-linear optical compound that satisfies the generally formula
(xcexa3iMxcex1i1)Be2O5xe2x80x83xe2x80x83Formula 2
wherein M1 is a mono-valent metal ion; and wherein (xcexa3ixcex1i)=X=6
Still yet another object of the present invention is to produce a non-linear optical compound that satisfies the generally formula
(xcexa3j=1-3Mxcex2j2)Be2O5xe2x80x83xe2x80x83Formula 3
wherein M2 is a di-valent metal ion; and wherein (xcexa3jxcex2j)=Y=3.
A further object of the present invention is to produce a non-linear optical compound that satisfies the generally formula
(xcexa3kMxcex3k3 )Be2O5xe2x80x83xe2x80x83Formula 4
wherein M3 is a tri-valent metal ion; and wherein (xcexa3kxcex3k)=Z=2.
The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its structure and its operation together with the additional objects and advantages thereof, will best be understood from the following description of the preferred embodiment of the present invention when read in conjunction with the accompanying drawing. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the applicable art or arts. If any other meaning is intended, the specification will specifically state that a special meaning is being applied to a word or phrase. Likewise, the use of the words xe2x80x9cfunctionxe2x80x9d or xe2x80x9cmeansxe2x80x9d in the Description of Preferred Embodiments is not intended to indicate a desire to invoke the special provision of 35 U.S.C. xc2xa7112, paragraph 6 to define the invention. To the contrary, if the provisions of 35 U.S.C. xc2xa7112, paragraph 6, are sought to be invoked to define the invention(s), the claims will specifically state the phrases xe2x80x9cmeans forxe2x80x9d or xe2x80x9cstep forxe2x80x9d and a function, without also reciting in such phrases any structure, material, or act in support of the function. Even when the claims recite a xe2x80x9cmeans forxe2x80x9d or xe2x80x9cstep forxe2x80x9d performing a function, if they also recite any structure, material or acts in support of that means of step, then the intention is not to invoke the provisions of 35 U.S.C. xc2xa7112, paragraph 6. Moreover, even if the provisions of 35 U.S.C. xc2xa7112, paragraph 6, are invoked to define the inventions, it is intended that the inventions not be limited only to the specific structure, material or acts that are described in the preferred embodiments, but in addition, include any and all structures, materials or acts that perform the claimed function, along with any and all known or later-developed equivalent structures, materials or acts for performing the claimed function.